A fuel cell acts to make a hydrogen-containing fuel gas and an oxygen-containing oxidizer gas such as air electrochemically react with each other to produce electric power and heat concurrently. With regard to its structure, a catalytic reaction layer mainly composed of carbon powder carrying thereon a platinum-based metal catalyst is formed on both surfaces of a polyelectrolyte membrane which selectively transports hydrogen ions. A gas diffusion layer (e.g., carbon paper, carbon cloth, etc.) having both gas permeability and electron conductivity is formed on an outer surface of the catalytic reaction layer. This diffusion layer and the catalytic reaction layer (catalytic layer) are combined to provide electrodes. Generally, an electrode to which hydrogen is introduced is called anode (hydrogen electrode, or fuel electrode) while an electrode to which oxygen is introduced is called cathode (oxygen electrode, or air electrode).
Next, in order that fed fuel gas or oxidizer gas will not leak or that the fuel gas and the oxidizer gas will not mix together, a gas sealing material or gasket is placed around the electrodes so as to sandwich the polyelectrolyte membrane. This sealing material or gasket is to be integrally assembled preliminarily with the electrodes and the polyelectrolyte membrane, such an integrated assembly being called MEA (Membrane Electrode Assembly). Outside the MEA, separators are placed for mechanical fixation of the MEA and for electrical series connection of neighboring MEAs to each other. At portions of each separator at which it contacts the MEA, gas passages are formed to serve for the supply of reactant gas to electrode surfaces and for carry-off of produced gas and excess gas. Such gas passages may also be provided independently of the separators, but it is a common method that recessed groove portions are provided on the surfaces of the separators to serve as the gas passages.
The supply of fuel gas to this recessed portion requires piping jigs for branching piping for use of fuel gas supply into the number of separators in use and connecting their branching destinations directly into the recessed portions used for the formation of gas passages of the separators. These jigs are called manifolds, and one type of them that is connected directly from the above-described fuel gas supply piping is called external manifold. The manifold includes a type called internal manifold, which is simpler in structure. The internal manifold refers to those which are so designed to supply fuel gas directly to a hole provided in the separator having gas passages formed therein with inlet/outlet ports of the gas passages passing up to the hole.
Since the fuel cell generates heat during operation, the cells need to be cooled by cooling water or the like so as to be maintained in a successful temperature state. As a cooling part for giving a flow of cooling water in units of 1 to 3 cells is normally inserted between one separator and another separator, it is often the case that cooling water passages as the cooling part are provided in rear faces of the separators to provide the cooling water. These MEA and separator and the cooling part are stacked one on another alternately until, for example, 10 to 400 cells are stacked, and thereafter the stack is sandwiched by end plates via a current collector plate and an insulating plate, and further fixed from both ends with tightening bolts. This is a common structure of the stacked fuel cell (i.e., fuel cell stack).
The separator to be used for such fuel cells, e.g. PEFCs (Polymer Electrolyte Fuel Cells) needs to have high electrical conductivity, high airtightness to fuel gas, and high corrosion resistance, i.e. acid resistance to reactions involved in oxidation-reduction of hydrogen/oxygen. For these reasons, in the making of conventional separators, the groove portions to form the gas passages are formed on the surfaces of a glassy carbon plate or resin-impregnated graphite plate or the like by cutting work, or expanded graphite powder is set together with a binder into a press mold with the gas-passage use groove portions formed therein and then subjected to press working and heat treatment or the like.
Also, in recent years, an attempt to use stainless or other metal plates instead of conventionally used carbon material has been made. In the case of a separator using a metal plate, since the metal plate is exposed to a high-temperature oxidative atmosphere, corrosion or dissolution of the metal plate may occur over long-time use. Corrosion of the metal plate would cause the corroded portion to increase in electrical resistance, resulting in lowered power of the cell. Also, dissolution of the metal plate would cause dissolved metal ions to diffuse in the polyelectrolyte, those metal ions being trapped by ion-exchange sites of the polyelectrolyte. As a result, the ion conductivity of the polyelectrolyte itself would deteriorate. In order to avoid such deteriorations, it is common practice that the surface of the metal plate is gold-plated to some extent of thickness.
With the PEFC, it is generally practiced that a gas containing hydrogen as a fuel gas or a gas containing oxygen gas as an oxidizer gas is mixed with steam and supplied as such so that hydrogen ionized within the polyelectrolyte is mobilized. Meanwhile, because moisture (steam) is produced by combustion reaction during power generation, steam that mixes with the fuel or the oxidizer and moisture (steam) produced by power generation pass through the passage-formation groove portion formed in the separator. Although the surface of the separator in the inner wall surface of the groove portion is generally controlled to a constant temperature so that such steam or produced steam will not be condensed more than necessary, yet variations in the power consumption of generated power or in the fuel supply would cause the amount of generated heat inside the fuel cell to vary, so that the internal temperature would be varied or the amount of produced water would be vary.
For instance, in the case of lowered temperature or the like, the separator surface (inner wall surface of the groove portion) may be more liable to condensation, and it is practically impossible to completely eliminate such a phenomenon. With occurrence of condensation, there is a problem of occurrence of a voltage instability phenomenon (flooding) that water drops generated by the condensation block the gas passages, causing deficiencies of fuel supply to the electrodes and the catalyst located downstream of the place of the blockage, in which case the voltage decreases gradually, and further, discharge of the water drops causes the blockage of the gas passages to be released, so that the fuel supply is recovered, in which case the voltage is increased.
As the material of the conventional-type fuel-cell separator, impermeabilized matters processed from graphite blocks, anti-corrosion metals, or liquid resin-containing matters obtained from an expanded graphite sheet molded laminate impregnated and hardened with liquid resin are usually employed, but separators made of such materials are poor at hydrophilicity. Therefore, a method has conventionally been used that the hydrophilicity at the separator surface (i.e., inner wall surface of the groove portion) is improved so that the contact angle between water drops generated within the gas passages and the inner wall surfaces of the groove portions is lessened to suppress the growth of the water drops that block the gas passages and thereby prevent the blockage of the passages.